The present invention relates to a semiconductor device of resin-molded type having a high reliability (e.g., a high moisture resisting property).
A semiconductor device of resin-molded type, in which a dye- or wire-bonded semiconductor element is molded and sealed (or packaged) upon a lead frame by means of a resin such as epoxy resin, has the advantages that its sealing construction is simple and that it can be fabricated at a low cost. However, such resin-molded semiconductor device has the disadvantage that its moisture resistance is inferior to that of a ceramic type package. More specifically, since such a resin will allow moisture to pass therethrough, the moisture outside of the package passes through the resin into the package so that it wets and corrodes the bonding pad of aluminum, which is formed on the surface of a semiconductor element, until the electrical connection breaks or deteriorates, thereby to shorten the life of and degrade the reliability of the semiconductor device.
With this in mind, the inventors proposed forming the surface of the semiconductor element of a resin-molded type package with a nitride film as a passivation film so that the moisture having passed through the resin will not wet the surface of the semiconductor element. For instance, FIG. 1 shows an example of that proposal, wherein a silicon oxide (SiO.sub.2) film 2 and an aluminum pad 3 are formed upon a semiconductor element 1 made primarily of silicon (Si), a phosphor glass (phospho-silicate glass) (PSG) film 4 is formed on the silicon oxide film 2 and the aluminum pad 3, a nitride (Si.sub.3 N.sub.4) film 5 is formed on the phosphor glass film 4, and the formed structure is molded with a resin 6. By the formation of such nitride film 5, the moisture having passed through the resin 6, if any, is blocked from passing into the nitride film 5 so that the moisture can be prevented from directly wetting the surface of the semiconductor element 1.
However, it has been found that a sufficiently satisfactory reliability (or moisture resisting property) cannot be obtained even if the semiconductor device is constructed in the manner thus far described. The reason therefor will be described in the following. Because of insufficient adherence between the resin 6 and the nitride film 5, there is established between the resin 6 and the nitride film 5 a clearance, in which the moisture which has passed through the resin 6 collects. Moreover, since, in case of removing a portion of the nitride film 5 on the pad 3 to expose a bonding area of the pad 3 prior to the bonding step, the bonding area must be made larger than the contact area between the wire 7 and the pad 3 considering the error of the alignment between the pad and the wire in bonding the wire, a portion of the bonding area is exposed to the resin 6 after bonding the wire 7, and the nitride film 5 has its end face positioned in the vicinity of the pad 3. As a result, either the moisture stored in the clearance between the nitride film and the resin or the moisture passing through the clearance moves from the end face of the nitride film up to the surface of the pad until it corrodes the pad. Especially in the case where the phosphor glass film 4 is used, as shown in FIG. 1, the glass film 4 has its end face exposed to the pad 3. As a result, the moisture having passed through the resin and the aforementioned exposed phosphor glass film react to produce phosphoric acid, by which the aforementioned pad is corroded. In order to prevent this, it is sufficient to form the nitride film 5 on the wire and the pad, too. It is, however, almost impossible to form a nitride film under the condition in which the wire has been connected to the pad, because the nitride film is formed by the plasma method.
Incidentally, although the nitride film 5 is formed on the phosphor glass film 4 in the example shown in FIG. 1, a semiconductor device having the nitride film formed directly upon the silicon oxide film 2 likewise experiences corrosion of the bonding pad. In this instance, an impurity (in the form of ions) in the resin 6 and the moisture react to promote the aforementioned corrosion.